The invention relates to a method for producing a component from an air-hardenable steel having excellent forming properties, in particular for lightweight vehicles. The invention also relates to a component produced with the method according to the invention.
The term component is to be understood in the following as a component formed from a sheet-metal blank or tube by forming with a forming tool.
The hotly contested automotive marketplace requires the manufacturers to continuously search for solutions for lowering the fleet fuel consumption while simultaneously maintaining the highest possible comfort and protection for the occupants. On one hand, the weight savings of all vehicle components plays an important role; on the other hand, highly advantageous properties of the individual component under high static and dynamic stress during the operation and in the event of a crash are also important.
The suppliers attempt to address these demands by reducing the wall thicknesses by providing high-strength and ultra-high-strength steels while simultaneously improving the properties of the components during manufacture and in operation. Such steels must satisfy comparatively high demands relating to strength, elasticity, tenacity, energy absorption and machinability by, for example, cold-forming, welding and/or corrosion resistance.
For ensuring corrosion resistance, metallic coatings made of zinc, aluminum or corresponding alloys based on zinc or aluminum which may contain additional alloying elements, such as Mg or Si, may be considered.
In addition to the aforedescribed general requirements, ultra-high-strength steels should attain the following exemplary mechanical characteristic values:    Relbzw. Rp0,2: 700-1000 [MPa]    Rm: 800-1200 [MPa]    A80: ≧10[%] and/or    A5: ≧13[%].
In the past, for applications of the crash- or weight-optimized components, mostly conventional steels with a relatively large sheet-metal thickness, water-quenched high-strength small-grain steels, multiphase steels or alternative materials, such as aluminum, were used.
Disadvantageously, conventional steels have a high component weight. Disadvantages of alternative ultra-high-strength multiphase steels are their poor weldability and forming properties due to the high basic hardness. Water-quenched and hardened steels are expensive to manufacture and therefore uneconomical.
Air-hardenable steel materials have been developed as an alternative, which overcome the disadvantages of conventional steels by realizing the required material properties solely by cooling the steel in air, for example following a heat treatment of the component. After cold-forming or shaping, the air hardening state can be adjusted by way of a subsequent heat treatment.
DE 102 21 487 B4, EP 0 576 107 B1 and DE 44 46 709 A1 disclose air-hardenable steels which can in principle be used for vehicle components. DE 10 2004 053 620 A1 discloses an advanced air-hardenable steel with excellent forming and welding properties with the following composition (concentration in mass-%):    C 0.07 to ≦0.15    Al≦0.05    Si 0.15 to ≦0.30    Mn 1.60 to ≦2.10    P≦0.020    S≦0.010    N≦0.0150    Cr 0.50 to ≦1.0    Mo 0.30 to ≦0.60    Ti 0.010 to ≦0.050    V 0.12 to ≦0.20    B 0.0015 to ≦0.0040    remainder iron, including typical elements in steel production.
The manufacture of components produced by quenching of press-hardenable steels in a forming tool is known from DE 601 19 826 T2. A sheet-metal blank which was previously heated to a temperature of θblank=800 to 1200° C. and provided with a metallic coating of zinc or based on zinc is formed into a component in an optionally cooled forming tool, wherein for attaining the required strength the metal sheet or the component is subjected during the forming process in the forming tool to quench-hardening (press-hardening) through rapid heat removal.
It has been observed in experiments that for attaining a desired tensile strength, the component must be subjected to subsequent annealing. This is complex and expensive and in addition reduces again the strength of the hardened component.
It has also been recognized in these experiments that components made of air-hardenable steels cannot be produced with the process disclosed in DE 601 19 826 T2, because the required elongation in the formed component can also not be attained with the quenching process.
It was therefore the object of the invention to provide a method for producing components made of air-hardenable steels with a forming tool, wherein the required mechanical properties on the formed component can be safely maintained while eliminating a final annealing step.